Carbon-based nanofillers represented by carbon nanotube (CNT) are excellent in thermal conductive properties, electrical conductive properties, mechanical properties and the like. Accordingly, addition of such a nanofiller to a resin have been examined to provide the resin with these properties or to improve these properties of the resin. Among such resin properties which are improved by the addition of such a carbon-based nanofiller, electrical conductive properties can be improved relatively easily; however, it is not easy to significantly improve thermal conductive properties.
In this connection, as a resin composition with improved thermal conductive properties, Japanese Unexamined Patent Application Publication No. 2005-54094 (PTL 1) proposes a thermally conductive resin material obtained by dispersing carbon such as fibrous carbon in a mixture of two or more resins. In this resin material, the carbon is dispersed selectively only in one resin phase. Thereby, a thermally conductive path is formed of the carbon, to improve the thermal conductive properties. Such a thermally conductive path also acts as an electrically conductive path, thereby enhancing the electrical conductive properties as well. For this reason, this resin material is not applicable to applications where both thermal conductive properties and insulating properties are required.
Japanese Unexamined Patent Application Publication No. 2005-150362 (PTL 2) discloses a highly thermally conductive sheet in which a thermally conductive filler such as carbon nanotube is dispersed in a commodity resin, and also discloses that an electrically insulating material such as alumina is dispersed in the commodity resin to reduce the electrical conductive properties of the sheet and to prevent an electrical short. However, even when an electrically insulating material such as alumina is dispersed, high insulating properties tend not to be achieved. Moreover, when a large amount of electrically insulating material is blended in the commodity resin for the purpose of improving the insulating properties of the sheet, the sheet tends to have a high relative density because the electrically insulating material has a high relative density, and to also have poor formability.
Meanwhile, in the application fields such as various parts for vehicles and various parts for electrical and electronic equipments, particularly in the application fields of various parts made of resin for automobiles, lighter and higher-functional materials are demanded from the viewpoints of reducing carbon dioxide emission and of saving energy. In this connection, there is a demand for resin materials which have both thermal conductive properties and insulating properties as well as a relative density in which an increase is restrained.